A police statement said the Turkish nationals — eight men and a woman — were detained in the central Athens areas of Neos Kosmos and Kallithea earlier Tuesday.

The operation was not connected with suspected Islamist terrorism, police said. The detainees are suspected of links with the Revolutionary People's Liberation Party-Front, or DHKP-C, deemed a terrorist organization by Turkey, the United States and the European Union.

Police said that searches of three homes where the Turks were detained produced detonators and materials "available on the market that could potentially be used to make explosives."

Erdogan's visit, announced Tuesday, is set for Dec. 7-8.

In 2014, four Turkish men were arrested in Athens on terrorism-related offences in connection with DHKP-C, after a raid on an Athens apartment that uncovered weapons, explosives and detonators. The operation followed the arrests of five Turks and three Greeks over a speedboat carrying arms that was intercepted in the Aegean Sea.

Originally founded in the late 1970s as Dev Sol, the Marxist-Leninist DHKP-C is believed responsible for a string of assassinations and bombings in Turkey, including a 2013 suicide bomb attack on the U.S. Embassy in Ankara.

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